1. Field of the Invention
The present invention relates to a speed change device that is capable of changing speed and torque between an input member and an output member, and also relates to a steering system using the speed change device to change a steering angle ratio thereof.
2. Description of the Related Art
A steering system with a speed change mechanism of this kind is disclosed in Japanese examined patent publication No. (Sho) 56-45824. This steering system has a speed change device disposed between an input shaft connected with a steering shaft and an output shaft connected with steering gears. The speed change device includes a first and second planetary gear sets arranged symmetrically to each other, and one of the sets can be controlled to change a transmission ratio. In the steering system, however, the planetary gear sets cause slapping gear noise and play between the adjacent gear teeth, impairing steering operation feeling. Improving the precision of a tooth configuration for avoiding these disadvantages brings high manufacturing cost and large resistance torque in steering operation.
Another variable steering system of this kind is disclosed in Japanese patent laying-open publication No. (Hei) 08-291848. This steering system is equipped with a speed change device consisting of two planetary roller sets disposed between an input shaft connected with a steering shaft and an output shaft connected with steering gears. However, the rollers cause a large slip therebetween in high torque range, decreasing transmitting torque. Using larger torque-transmittable rollers for reducing the slip amount brings large resistance torque in steering operation.
Incidentally, Japanese patent laying-open publication No. 2002-213566 discloses a speed change device having a planetary roller set and a planetary gear set that are arranged in parallel to each other between an input shaft and an output shaft. In this device, a sun roller is connected with a sun gear, planetary rollers are connected with planetary pinions, and a ring roller is connected with a ring gear so as to remove the above disadvantages of the conventional steering systems.
The above known conventional speed change device, however, encounters a problem that it is not suitable for a steering system of a motor vehicle. The reason comes from that the conventional speed change drive is constructed so that the rollers with the gears rotate continuously for a certain period in one rotational direction or the other rotational direction. This operation is different from an operation, such as in a steering operation of a motor vehicle.
The gears used in this conventional speed change device are formed to have backlash, which is set very small, so that the gears can rotate smoothly and high-efficiently during continuously rotating. When the above conventional speed change device is used in a steering system, the input to the gears are continually changed in its rotating direction and in the amount of torque, which causes the gears to easily and frequently engage with each other during the steering operation, even in a normal steering operation when the vehicle runs. This frequent engagement of the gears impairs a steering operation feeling of a driver.
It is, therefore, an object of the present invention to provide a speed change device which overcomes the foregoing drawbacks and can smoothly transmit torque under the circumstances such that input changes continuously or frequently in the amount of input torque and an input direction.
It is, therefore, another object of the present invention to provide a steering system which overcomes the foregoing drawbacks and can be operated smoothly in a normal steering operation and ensure high torque at variable steering angle in an exceptional steering operation, such when a motor vehicle is stopped and a steering wheel is rotated at a large angle.